190 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



claviers. The bellows are in action, and consequently the wind is in 

 the wind-trunk at the proper pressure. 



The organist begins by drawing out the stops he wishes to use 011 

 the several key-boards. These stops move a series of levers, which 

 open the corresponding registers or stops. 



That done, the pipe does not yet speak, although the wind- chests 

 of the sound-boards are filled with wind ready to do the work 

 wherever wanted. As soon as the organist places his finger on one 

 of the notes of one of the claviers, immediately a valve (s) inside the 

 wind-chest (AB) of a sound-board is opened, the wind penetrates into 

 the corresponding groove, and thence into the pipes with the stops 

 pulled out. The same thing happens if with his foot he presses 

 down one or other of the pedals. From this moment the organ is at 



FIG. 136. Transversal section of the sound-board. Wind chest and valve. 



work, and melodies as well as their harmonious accompaniments are 

 produced at the will of the performer. 



We have given the description of the organ and its mechanism as 

 it was built at the end of the last century from D'Alembert and 

 Diderot's great Encyclopaedia, where it is described at great length. 

 The reason of this is that many organs still existing are made after 

 this model. But during the last century organ-builders have arrived 

 at great perfection in detail corresponding to the progress of indus- 

 try and art during this period. The mechanism of this wonderful 



